Electric discharge device



July 18, 1944. D. D. HINMAN 2,353,668

ELECTRIC DISCHARGE DEVICE Filed Oct. 5. 1942 Figi.

Invvenftor':

l His Afttorneg.

Donald D. Hinman,

Patented `luly 18, 1944 ELECTRIG DISCHARGE DEVICE Donald D. Hinman, Cleveland Heights, Ohio, assignor to General Electric Company, a corporation of New York Application October 5, 1942, Serial No. 460,913

6 Claims.

This invention relates to electric discharge l devices useful as sources of radiation for various purposes, including ultraviolet rays as well as visible light. It is particularly useful for purposes requiring a concentrated source f rather high intrinsic brilliance, including projection lamps for visible light or ultraviolet, or for both. The invention overcomes difculties which arise when the considerable energy required for high brilliance is to be dissipated in the relatively small tubular discharge chamber or space of a lamp of high pressure type, such as exemplified by the capillary lamps marketed under the designations H-3 and H-6. Lamps of this type containmercury or other vaporizable and ionizable metal as the principal Working substance, (usually with an ionizable starting gas like one of the inert rare gases), and operate under a vapor pressure that is at least suflicient to constrict the discharge or arc into a linear stream through the axial region of the discharge space. In. order to limit the pressure and the voltage uctuation in the lamp, and for various other reasons, it is desirable to operate it with the mercury or other vaporizable working substance4 completely vaporized. As contrasted with the recognized low pressure type of lamp, in which the mercury vapor pressure during operation may be measured in microns, rarely exceeding 1 mm., the type of lamps distinguished as high pressure have operating pressures of from about 1/2 atmosphere upward, sometimes ranging over 100 atmospheres, with the possibility of much higher pressures.

The brightness of a vapor arc is-largely a function of the energy dissipated in the arc per unit of area of the inner surface of the discharge chamber or envelope. As part of the energy in the discharge is always converted into heat instead of light or other desired radiation, the.

maximum brightness attainable in a lamp is limited by the ability of its envelope to withstand the high temperature thus produced by the discharge. Fused quartz being about the most refractory light and radiation-transmitting material practically available, its softening Dfint determines a denite limit for the temperature that the lamp envelope will stand; and a somewhat lower practical limit is imposed by the increasing rate of devitrication of quartz as the temperature increases. When seal glass is `employed in the envelope construcfion, as well as quartz, its properties may also impose temperature limitations. These considerations correspondingly restrict the permissible energy input into a given lamp, and its intrinsic brilliance, under any given conditions of external coolingof the lamp envelope or bulb.

For quartz lamps of the type vabove referred to, an energy input of some 35 watts per square centimeter of internal wall areakis about the practical limit with mere convection cooling by the surrounding air of the ordinary indoors atmosphere at about 20 C. With the high forced cooling of the lamp envelope that is attainable by circulation of water or of high pressure air, very much higher wattage input per square centimeter becomes possible, such as some 600 watts, more or less. However, these high intensity lamps have the drawback that it is not always practicable or convenient to provide water or compressed air for such high forced cooling, more especially since the power required to furnish compressed air cooling may be about twice that consumed by the lamp itself. On the other hand, there are many uses for high intensity lamps of intrinsic brilliance corresponding to intermediate wattage inputs (relative to wall area), if they could be operated without the high cooling for which water or high pressure air is necessary.

It hasbeen sought to meet the need for high intensity lamps, of higher wattage than is possible with mere atmospheric convection cooling, by forced cooling with a blast of low-pressure air blowing directly on the lamp envelope. This, however, gives rise to other difficulties and complications, especially in the case of lamps intended to operate with their mercury or .vaporizable working substance completely vapcrized. With such a lamp, all parts of the envelope must be kept below the linut imposed by the properties of quartz or of any seal-glass that may enter into the envelope construction; and yetv the lowest internal temperature anywhereA` in the lamp must be high enough to prevent con` densation of mercury vapor at the desired operating pressure, since otherwise this pressure and the desired lamp wattage could not be maintained. These requirements must be fullled .when the lamp has been partially blackened in operation and its heat absorption thus increased, as well as when the lamp is new and entirely clean.

The main central portion of the tubular lamp envelope between the active electrode tips in its ends is directly and effectively heated by the arc stream, and can be adequately cooled (being short) by an air jet from a single nozzle laterally aimed at its middle. However, the spaces around and behind the electrodes (commonly called end chambers) are heated less directly and less effectively, and hence do not attain adequate temperatures under the air blast cooling that is required to prevent overheating of the mid-#portion of the lamp.- In an envelope tube of anything like usual capillary size of bore, the space requirements of the electrodes and of the ordinary'inlead seals prohibit a reduction of tube bore around them to make the tube ends warm up sufficiently; ,and even if these ends were reduced in size somewhat, the resulting decrease in their internal heat dissipating surfaces would be more or less counterbalanced by the great concomitant reduction in annular space around the electrodes that would be available for thermal convection currents in the lamp atmosphere.

The case is further complicated by the fact that when the lamp is shut down, mercury condensesv mainly in the end chambers; so that when the lamp is restarted (no matter how long afterward), the initial vapor pressure in the lamp Vis low, and hence the energy input and the heat output for vaporizing the unfavorably situated condensed mercury are also low. Thus the lamp heats and comes up to full wattage very slowly; and with air blast cooling such as'just described, it would never heat up vor attain full pressure and wattage.

In my application Serial No. 385,721, led March 28, 1941, now Patent No. 2,321,678, granted June 15, 1943, assigned to the assignee of this application, I have shown how diiculties in starting or operating an air-blast cooled tubular lamp can be overcome vby reducing the natural loss or dissipation of heat from its ends, as by heat insulative wrappings of asbestos paper, or by cylindrical quartz skirts or shields .surrounding the end chambers and sealed to the quartz lamp tube at its inner end, adjacent the electrode tip, or by cylindrical metal skirts or shields similarly protecting the end chambers. Auxiliary electric resistance heaters may be applied around the lamp tube inside the quartz or metal skirts, and may be energized to preheat the tube ends before starting, or to heat them during operation, or both.

I have now discovered, however, that a small tubular lamp can be made to heat itself internally substantially or nearly uniformly throughout, even when cooled by a low pressure air blast as above described, without any such provisions for heating its ends, or for reducing the natural loss of heat from them. My present invention also allows of greatly reducing the warm-up time heretofore required for starting or restarting such a lamp. These and other features and advantages of the invention will be better understood from the following description of species and forms of embodiment. and from the drawing.

In the drawing, Fig. 1 is an enlarged longitudinal sectional view of a quartz tubular or capillary lamp embodying my invention, with a wiring diagram of suitable electric circuit connections, also showing a cooling air jet nozzle pipe; Fig. 2 is a Side view of an envelope blank suitable for the fabrication of the lamp, with the inlar envelope I0 of fused quartz having a substantially cylindrical form, except at its end chambers II, II, and permeable toultraviolet and visible radiation. At the opposite envelope ends II, II are shown solid operating electrodes I2, I2 attached to the inner ends of axial inleads I3, I3 which are sealed through vitreous external cylindrical end seal extensions I4, I 4-conven- 'iently made of a size approximating the external envelope diameter. The electrodes I2, I2

may be of unactivated refractory metal such as the discharge space or cavity in the envelope lead and electrode assemblies in place but not yet fused in, the scale being smaller than for Fig. 1; and Fig. 3 is a side view on a stillv smaller scale, illustrating the mounting of the lamp in a reflector.

As shown in Fig. 1, the discharge device is a lamp L comprising a vitreous elongated or tubulll represent a constriction of the envelope such as to result in a discharge-constricting pressure therein during normal operation, and the amount of mercury I 5 may be such as to give an unsaturated mercury atmosphere of some 5 to 10 atmospheres pressure at normal operating temperature. The cavity of the envelope lll should be without sharp corners or any irregularities sumcient to form recesses Where relatively ineffective heating from the arc stream might allow mercury to condense. Accordingly, the angular hollow corners that would exist at the ends of the envelope i0 if they were iiat are filled out and rounded away. As shown in Fig. l, this reduction of the hollow corners preferably exceeds what would make the envelope end chamber I I hemispherical merely; and the tube end slopes in toward and around the electrode I2, in a conoidal form-whose profile consists of 'long arcs of radii decidedly greater than that of the tube bore and an intermediate arc of radius less than that of the bore. The electrodes I2, I2 are coaxial with the tube bore and approximately cover these rounded apices of the end chambers I I, II. though they are slightly spaced from the end surfaces, with their inner ends about in the planes where the rounded end chambers merge with the cylindrical tube bore.

Contrary to usual practice in capillary lamps having solid electrodes, the electrodes I2, I2 are ,mounted close up against the envelope ends I I,

Il, almost touching the end Walls, a'nd are of such compact, squat proportions that they project relatively little into the end chambers of the envelope. -Such close relations of the electrodes I2, I2 to the envelope ends II, II are important in order to secure adequate heating of the end chambers around the electrodes 'by heat from the arc stream; for this arrangement not only results in minimum obstruction to the endchamber convection currents in the mercury vapor (due-to the heat of the are stream), but also in heating of the end chamber walls by radiation from the electrodes and by metallic heat conduction through the inleads I5, I5. The optimum in this respect is that the end chamber Walls should be everywhere as close to the electrodes I2, I2 `as consists with substantially equal heating of all points of these walls-both from the electrodes and from the arc stream-and this is favorably approximated by the conoidal forms of the end chambers here shown. Also very important to the heating of the end chambers is the unactvated character of the electrodes I2, I2, as

will be explained hereinafter. As shown, each of the electrodes I2, I2 is constructed out of a. helical coil of a few turns of tungsten wire, comparable in gauge to the inlead wire I1 and prefera'bly woundwith its own convolutions in lateral contact, flttecbaround an inner inlead section I1 of tungsten wire and preferably welded thereto.

In order to take full advantage of the substantially uniform internal heating of the lamp enyelope I and operate the lamp at the highest wattage consistent with the mode of cooling employed, it is preferred to make the lamp envelope entirely of one refractory vitreous material, such' as quartz, without the employment of special lower-melting material at the end seals I4, I4. l

While quartz end seals around circular tungsten inlead wires have not proved satisfactory-showing objectionable leakagethis can be overcome by using ribbon type end seals such as shown in Patent 2,094,694 to Bol. As shown in Fig. 1, each seal I4 may comprise a length of molybdenum ribbon I8 whose ends are laterally welded to inner and outer lead wire sections I1 and I9, the former of tungsten and carrying an electrode I2,

the latter of tungsten or molybdenum. In Fig.

1, the ribbons I8, I8 of the two seals, Il are shown as lying in planes at right angles to one another, though this is not at all essential. To obviate any dimculty in welding the wires I1, I9 to extremely thin ribbon I8 without either melting the latter or failing-to produce joints of adequate conductivity, it is desirable to reinforce or thicken the ends of the ribbon I8, which may be done by solidly welding on refractory sheet metal facings 28, preferably at both sides of the ribbon I8. As shown in Fig. 1, a short length of molybdenum ribbon. preferably thicker than the ribbon I8, may be folded around each end of the latter to provide its two facings 20, 2l). After these facings 20, 28 have been welded on more or less solidly-or at least at numerous' points over the width and area of contact-the resulting virtually integral thickened ends ofthe ribbon I8 can be laterally welded rml'y and solidly to the lead wires I1, I9 without difficulty. The nal result is a lead and electrode assembly E such as shown in Fig. 1.

If found more convenient, of course, this assembly E may be made up without the electrode I2, and the latter welded on the lead section I1 as afinalstep.

In fabricating the lamp, a quartz envelope or bulb such as shown in Fig. 2 may be made, consisting of a main tubular portion corresponding essentially tothe envelope I 0 as shown in Fig. 1, with integral long, heavy-walled, end tubes T, T, just large enough to freely pass the lead and electrode assemblies E, E, and with a lateral exhaust tube t open into the envelope I0. The assemblies E, E are slid in from each end until the electrodes I2, I2 are properly positioned-as shown in Fig. 2-in correspondence to Fig. 1. The tubes T, T having been sealed by fusion at their outer ends, as indicated in dot and dash lines in Fig. 2, the device is evacuated through the tube t andthe latter is also sealed olf at some distance from the bulb B,as indicated in dot andV dash lines. The assemblies E, E being properly positioned, and protected against oxidation by the vacuum, each end tube T is fused and collapsed around its inlead I as indi' cated in dot and dash lines in Fig. 2, forming the rounded end chambers I I, I I and embedding and ensealing the inner leads I1, I1 and the ribbons I8, I8. The tube t is then reopened, and the device is again and finally exhausted through it, dosed with a suitable amount of mercury for\its charge I5, and also charged with its filling of starting gas at suitable pressure, as argon at 20 mm. The tube t then is sealed or tipped off very close to the bulb- B, as indicated in dot and dash lines in Fig. 2, all according to usual capillary lamp practice. Finally, the exhaust seal tip is heated and fused just enough to allow it to be forced in by the pressure of the surrounding atmosphere, thus filling the usual exhaust tip recess and producing a somewhat irregular conformation, that is suggested at 2| in Fig. l. When the end tubes T, T are cut off beyond the fused zones, the main part B is left with the end seals Il, I4 shown in Fig. 1.

In exhausting the device the second time, it is desirable to heat it in an oven to a temperature of some 1200 C. in order 4to degas the internal walls of the discharge space. When this is done before the sealed'tube ends T, T are cut olf, these tube ends protect the lead sections I9, I9 from oxidation in the oven. After removal of the device from the oven, though before sealing off at t from the exhaust system, it is desirable to pass current between the electrodes I2, I2 in order to degas them thoroughly. For this purpose, a sheet metal sleeve (not shown) may be slipped around each of the sealed end extensions T, T and its sealed-in lead I3, and the terminals of a high-frequency spark coil may be connected to these sleeves, thus providing capacity couplings to the electrodes I2, I2. The A. C. discharge current passed between the electrodes I2, I2 should be as high as can be used without producing blackening of the envelope visibly. l

I do not herein claim the temporary sealing up of the tube ends T, T while the leads I3, I3

are being sealed in and until after the'final exhaust, norithe mode of exhausting the tube and degassing the electrodes II. II, etc., since all this is the invention of Edward B. Noel.

As shown in Fig. 3, the,v lamp L may be supported from the outbent ends of current supply leads 22, 22 by means of clips 25 of'flat strip metal embracing the end seals I4, I4 and the leads 22, 22, and welded to` the latter. AShort connecting wires 26, 26 may be welded to the outer lamp leads I9, I9 'and to the current supply leads 22, 22. Cooling air may be blown against one side of the lamp envelope I0 at its mid-length through a nozzle -pipe 21 located in close proximity thereto and supplied by a suitably located blower 28. When the lamp L is used with a parabolic reflector R as a projector, it may be mounted at the reector focus F so as to extend at right angles'to the reflector axis, and the pipe 21 may be brought in through an axial hole in the reflector, While the leads 22, 22 `may be brought in through other, openings in the reflector,.to which they may be attached as in seal beam headlight lamps. If the front of the reflector R is closed in by a cover glass or lens (not shown), ample provision should of course be made for escape of the warmedv cooling air.

A suitable operating and starting circuit for the lamp L is illustrated in Fig. 1 as, comprising a step-up auto-transformer A Whose secondary is connected across the leads 22, 22, while its primary is connected across a supply line 30. One of the leads 22 includes a choke coil or ballast 3|, while the other includes the switch-opening electro magnetic coil 33 of a normally closed and selfy closing relay vacuum switch 34 in a shuntl circuit '35 connected across the leads 22, 22. A control switch 36 is shown connected in one side of the` supply line 30. When the control switch 36 is closed to energize the lamp supply circuit 22, 22,

the relay switch 34 opens suddenly, producing a high-voltage surge across the discharge gap between the lamp electrodes I2, I 2 that initiates trodes from which it takes oil", and this heat is partly transmitted back through the electrodes I2, I2 and their leads I3, I3 to the walls of the 4end chambers II, II. Heat is also transmitted from the electrodes I2, I2 to the end chamber walls by radiation and by gaseous or vaporous conduction and convection. Thus the discharge maintains the electrodes II, II at a temperature of ample electron emission which obviates serious sputtering of the electrodes, and hot enough throughout to keep the end chambers II, II from anywhere falling below a temperature corresponding to the desired mercury pressure. At the same time, the very large radiation of energy from the very hot electrodes I2, I2 (which. increases according to the fourth power of their temperature, measured "on the Kelvin scale) prevents them from attain- .ing a temperature at which the metal would vvaporize readily and seriously blacken the encombination with the low thermal conduction of.

the silica of the end seals I4, I4, the low thermal conduction of the thin ribbon lead sections Il, I8 limits the loss of heat along the leads I3, I3, and

requires an air pressure of about 11/2 pounds per surface. The output is rich in ultraviolet of 3650 A. wavelength. Such a lamp may be built with an envelope I0 of clear fused quartz having a discharge cavity about 22 mm. long and 3.5 mm. in diameter, and an external diameter of about 6 mm., while its discharge gap between the proximate' tips of the electrodes I2, I2 may be 18 mm. A shorter gap gives a better light beam shape, but gives a shorter life or a longer warm-up timeunless more cooling air is used. The internal diameter of 3.5 mm. is also a practical optimum: a larger diameter gives a slower warm-up, while a smaller size means a shorter useful life before excessive blackening occurs. The inner and outer leadwire sections I1, I9 may be of 20 mil tungsten wire with lengths of 5 mm. and 8 mm. respectively, while the intermediate ribbon section I8 may be of molybdenum 0.6 mil thick, 62 mils wide, and 10 mm. long. The reinforcing facings 20, 20 for each end of the ribbon I8 may be made up from a Astrip of molybdenum ribbon l mil thick, 62 mils wide, and 3 mm. long, which is rst bent to a V and then placed on the end of the ribbon I8 and closed iiat upon the latter preparatory to welding. The coil forming each electrode I2 may consist of three close turns of 20 mil tungsten wire wound at 100 per cent pitch on a mandrel wire of the same size; and after being set by suitable heat, it may be slipped on the end of the lead section I'I and welded fast in a. protective inert or reducing atmosphere. The metal charge I5 may consist of a total of 1.8 mg. of mercury. Y

For cooling the lamp, an air jet may be blown on it from a lateral nozzle pipe 21 of ai! inch bore aimed directly and perpendicularly at the midlength of the discharge cavity and located about 1 cm. from the axis of the lamp envelope I Il, with which the nozzle is axially aligned. An air flow of 2 cubic feet per minute is satisfactory, and

' square inch (above atmospheric) directly behind thus contributes to 'the' adequate heating of the end chambers II, I I and to maintaining the electrodes I2, I2 at a temperature of adequate thermionic emission. 'I'he rough or interstitious surfaces of each electrode I2 (due to the V-grooves between its coil convolutions) greatly facilitate the starting of the arc discharge, making it possible to start at a much lower effective voltage across the arc gap. The squat configuration of the electrode I2 and its small mass and thermal capacity enable it to be heated throughout very quickly when the discharge is started, so as to become adequately lemissive from its arcing area. before appreciable sputtering can occur. For the convenience of those desiring to practice the invention, specific construction data are given for a tubular lamp and accessories such as illustrated in Figs. 1,-3 suitable for operation with the nozzle opening. The cooling air is externally jetted against the envelope substantially symmetrically with respect to its mid-length, which is the middle of the' discharge gap between the electrodes I2, I2, being distributed to corresponding points of its two ends substantially in equal quantities and at equal velocities, so as to cool the two halves (and, in particular the tWo end cham-bers II, II) `essentially alike. Owing to the closeness of the nozzle opening 21 to the envelope and to the fact that the opening is of smaller diameter than the external diameter of the envelope III, the constricted air stream has a width substantially less than the envelope width presented to the impinging air, and acts to erode or"bufi awaythe layer or skin of warm air -that tends to cling to the envelope surface, thus cooling the envelope very energetically and effectively.

With this cooling, the lamp warms up in from 9 to 15 seconds, and restarts in less than 5 seconds when switched oil' while at normal operating temperature and then at once switched on again. When a reflector R is used, it may be an ordinary '7 inch glass headlight reflector (of an energy input of some 400 watts and a discharge current of about 3.6 amperes from a supply line 30 of 115 volts, for example, to yield 17,100 lumens or 1660 candlepower, with a brightness of 64 candles per square millimeter of internal envelope sealed beam type), and may be used without any lens or cover glass. While devitrication'of the quartz envelope I0 is somewhat slower when the jet pipe is above the envelope I0 as shown in Fig. 1, rather than below, the lamp performance is in other respects practically the same for all D0`- sitions of the jet about the lamp, so that the air jet may blow on the envelope I0 from any side thereof, according to convenience or the exigencies of service; and the lamp operates equally well in all positions from vertical to horizontal.

The lamp may be operated from a 115 volt A. C. line '.'illy by means of a transformer A such as commonly used for the mercury vapor lamp marketed under the commercial designation 400- watt type H1 mercury vapor lamp, giving an open circuit secondary voltage of about 240 volts R. M. S., vand a starting kick across the arc gap of the order of 2000 volts. The inductive ballast 3l may also bethat commonly used for this H1 lamp, and the starting switch 34 may be a shorter switch such as the fast action vacuum magnetic switch commonly furnished for starting commercial Cooper Hewitt lamps, and marketed under the designation vacuum shifter assembly."

' What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electric discharge device of tubular highintensity, high-pressure, cold-starting, self-heating solid electrode type, operable with external forced air cooling at an energy input relative to its internal area at which its envelope would soften if uncooled; said device comprising in combination a refractory tubular radiationtransmitting envelope having end chambers with corners rounded away, and of suillciently small internal diameter to be internally heated between said end chambers, by the arc stream due to said rated energy input, at least to a temperature corresponding to the desired mercury pressure, notwithstanding external cooling as aforesaid, with ionizable starting gas in said envelope, and also a charge of mercury therein that is only suiilcient to .provide an unsaturated atmosphere under the aforestated internal heating and external cooling; and solid unactivated refractory metal electrodes in said end chambers, with unobstructed annular spaces around said electrodes in said end chambers, said electrodes being so small, and

having their mass concentrated so close to the envelope ends, that the discharge maintains these yunactivated electrodes thermionically emissive and hot enough throughout to keep the end chambers from anywhere falling below the temperature aforementioned, whereby said end chambers are free of condensed mercury during operation.

2. The combination with a discharge device as set forth in claim 1, of nozzle means for letting cooling air externally against said envelope, substantially symmetrically with respect to its midlength.

3. An electric discharge device of tubular highintensity, high-pressure, cold-starting, self-heating solid electrode type, operable with external forced air cooling at an energy input relative to its internal area at which its envelope would soften if uncooled; said device comprising in combination a refractory tubular radiation-transmitting envelope having conoidal end chambers and v of a sufficiently smal1 internal diameter to be internally heated between said end chambers, by the arc stream due to said rated energy input, at least to a temperaturev corresponding to the desired mercury pressure, notwithstanding external cooling as aforesaid, with ionizable starting gas in said envelope, and also a charge of mercury therein that is only sufllcient to provide an unsaturated atmosphere under the aforestated their apices, with unobstructed annular spaces trodes thermionically emissive and hot enough throughout to keep the end chambers from anywhere falling below the temperature aforementioned, whereby said end chambers are free of condensed mercury during operation.

4. An electric discharge device of tubular highintensity, high-pressure, cold-starting, self-heating solid'electrode type, operable with external forced air cooling at an energy input relative to its internal area at which its envelope would soften if uncooled; said device comprising in combination a refractory tubular quartz envelope having end chambers with corners rounded away and quartz end extensions, and of sufllciently smal1 internal diameter to be internally heated between said end chambers, by the arc stream due to said rated energy input, at least to a temperature corresponding to the desired mercury pressure, notwithstanding external cooling as aforesaid, with ionizable startinggas in said envelope, and also a charge of mercury therein that is only sufllcient to provide an unsaturated atmosphere under the aforestated in- -ternal heating and external cooling; current connectors embedded and sealed in said quartz end extensions,.including thin ribbon seal sections that limit therma1 conduction via said current connectors, and also including unactivated tungsten inner sections entering said end charnf bers; and close-wound unactivated tungsten coils mounted on the inner ends of said inner lead sections substantially against the envelope ends, and coacting with said inner lead section ends to form compact unactivated electrodes with unobstructed annular spaces around them in said end c'nambers, said electrodes being so small, and having their mass concentrated so close to the envelope ends, that the discharge maintains these unactivated electrodes thermionically emissive and hot combination a refractory tubular quartz enve-v lope having end chambers with corners rounded away and quartz end extensions, and of sufficiently small internal diameter to be internally heated between said end chambers, by the arc stream due to said rated energy input, at least to a temperature corresponding to the desired mercury pressure, notwithstanding external cooling as aforesaid, with ionizable starting gas in said envelope, and also a charge of mercury therein that is only suicient to provide an unsaturated atmosphere under the aforestated in- I ternal heating and external cooling; current connectors embedded and sealed in said quartz end extensions, including thin ribbon seal sections internal heating and external cooling; and solid t rough-surfaced unactivated compact refractory metal electrodes of squat configuration in said.

that vlimit thermal conduction via said current connectors and have thickened ends, and outer and inner lead sections4 laterally welded to said thickened ends; and solid unactivated compact refractory metal electrodes in said end chambers attached to said connectors, with unobstructed annular spaces around said electrodes in said end chambers, said electrodes being ,so small, and having their mass concentrated so close to the envelope ends, that the .discharge maintains these -unactivate'd electrodes thermionically emissive soften if uncooled; said device comprising in combination a refractory tubular quartz envelope having end chambers with corners rounded away and quartz end extensions, and of suiilciently small internal diameter to be internally heated between said end chambers, by the arc stream due to said rated energy input, at least to a temperature corresponding to the desired mercury pressure, notwithstanding external cooling as aforesaid, with ionizable starting gas in saiden- Vvelope, and also a charge of mercury therein that is only sumcient to provide an unsaturated atmosphere under the aforestated internal heating and external cooling; current connectors embedl ded and sealed Vin said quartz end extensions, in-

cluding thin ribbon that limits thermal conduction via said current connectors, with thicker foil facings welded to each end of said ribbon at both sides thereof, and outer and inner lead sections l0 laterally Weldedv to facings at opposite ends of the ribbon; and, solid unactivated compact refractory metal electrodes in said end chambers attached to said connectors, with unobstructed annular spaces around said electrodes in said end l5 chambers, said electrodes being so small, and

having their mass concentrated so close to the envelope ends, that the discharge maintains these unactivated electrodes thermionically emissive, and hot enough throughout to keep the end 20 chambers from anywhere falling below the temperature aforementioned, whereby said end chambers are free of condensed mercury during operation.

DONALD D. HINMAN. 

